You could do a vinyl, aluminum or direct set. The glass should be tempered .

direct set would be cheapest since there is no dram and could be single glaze.

+1. Probably a direct set that is trimmed out with some aluminum or PVC trim. Needs to be tempered or laminated per code.

Thank you both - what is "direct set"? Does that mean no frame around the glass? Just a sheet of glass set in the tile or wall?

Yes, just a sheet of glass.

a vinyl or aluminum will window will be between 2 1/2 to 3 1/4 thick , the glass will be as thick as the glass, I'd go 3/16 minimum to 1/4 on that.

Direct will give the most options if you want anything decorative.

id go direct set if it was my house.

Call a glass company that builds shower enclosures. Ask for Todd